(Edit) Did the above only to find out that 5.3 doesn't work well with Drupal 6, Gallery2, etc.
Was able to downgrade back to PHP 5.2.6 then upgrade to 5.2.12.
aptitude install php5=5.2.6.dfsg.1-1+lenny4
aptitude install php5=5.2.12-0.dotdeb.1

Drupal is working fine now with PHP 5.2.12. But ISPConfig 3 admin is giving some errors. They don't really affect anything but probably should be addressed.

Code:

Strict Standards: ob_start() [function.ob-start]: It is not safe to rely on the system's timezone settings. Please use the date.timezone setting, the TZ environment variable or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'America/Los_Angeles' for 'PST/-8.0/no DST' instead in /usr/local/ispconfig/interface/lib/app.inc.php on line 35
Warning: ob_start() [ref.outcontrol]: output handler 'ob_gzhandler' conflicts with 'zlib output compression' in /usr/local/ispconfig/interface/lib/app.inc.php on line 35

EDIT: It seems zlib compression was enabled after the update and was causing this error. I disabled it and the error went away. Not sure how that happened.